1. Field of the Invention
The present invention relates to a writable or rewritable optical disc device, and more particularly, to an optical disk data reproducing apparatus which controls either directly or indirectly an automatic gain control circuit which unifies variations in signal amplitude, using an automatic laser power control (ALPC) period signal indicating that the laser power is being automatically adjusted.
2. Description of the Prior Art
The read signal from an optical disk is an intermittent signal containing no-signal periods, and the amplitude of the read signal may be changed by the frequency of the read signal or by the disc media itself. A conventional optical disk data reproducing apparatus typically has an automatic gain control circuit comprising an automatic gain adjustment function to smooth such amplitude changes in the read signal, and a gain hold function to hold the gain level constant during the no-signal period.
A conventional optical disk data reproducing apparatus is shown in FIG. 1. In FIG. 1, element 1 is an optoelectric conversion circuit; element 2 is an RF amplifier; element 3 is an automatic gain control circuit; element 4 is a signal presence detector; element 5 is a weak voltage signal; element 6 is an amplified signal; element 7 is a wave shaped signal; element 8 is a no-signal state gate signal; element 101 is a PIN diode; element 102, 103, 201; element 202 and 203 are resistors; element 204 is an operational amplifier, 301 is an automatic gain controller, 401 is a diode; element 403 is a capacitor, and element 404 is a comparator.
The optoelectric conversion circuit 1 receives a read light at the PIN diode 101 and converts the light to a weak current, which is converted to a weak voltage signal 5 by a current-voltage convertor circuit comprising resistors 102 and 103 and an operational amplifier 104. The weak voltage signal 5 is amplified by an RF amplifier 2, comprising resistors 201, 202 and 203 and operational amplifier 204. RF amplifier 2 produces an amplified signal 6. The amplified signal 6 is shaped by the automatic gain control circuit 3 comprising an automatic gain controller 301. The wave shaped signal 7 as produced from the automatic gain control circuit 3 is applied to a signal presence detector 4. The signal presence detector 4 includes an envelope detector comprising the diode 401, resistor 402, and capacitor 403, which produces an envelope detected signal indicating the change in amplitude of the wave shaped signal 7. The envelope detected signal and a threshold value are compared by the comparator 404, and when the envelope detected signal is less than the threshold value, a no-signal state gate signal 8 is set TRUE, indicating that the wave shaped signal is now in a no-signal state. The no-signal state gate signal 8 is input as the gain hold control signal of the automatic gain control circuit 3. Thus, when the no-signal state gate signal 8 is TRUE, the automatic gain control circuit 3 holds the gain to a predetermined constant value. This is to prohibit unnecessary signal amplification or attenuation to shape the amplitude of the wave shaped signal 7 regardless of whether the automatic gain control circuit 3 is in a no-signal state.
In general, the optical disc media contains both an address period to which is written position information identifying the location of data on the media, and the actual data storage period to which the actual data is written. In addition, there is an ALPC period provided in a writable optical disc media for adjustment of the laser power required for data writing. This ALPC period is generally provided between the address period and the data period. During the ALPC period, the laser power is kept witting power longer than the pulse length in the data period. As a result, heat builds up easily in the ALPC period, causing the crystal structure to change and deteriorate and media deformation to progress more rapidly, resulting in a higher and abnormal read signal amplitude.
However, because the signal amplitude in the ALPC period is not considered in a conventional device as described above, the problem exists that an inappropriate gain level is set for the read signal in the data period adjacent to the automatic laser power control because the automatic gain controller adjusts the gain based on the signal amplitude in the ALPC period.
Problems confronted with the prior art are described below with reference to the accompanying figures. In FIG. 2 is shown a graph of the read signal from the address period through the ALPC period and to the data period. In FIG. 2, waveform (a) is the amplified signal 6, which is the output signal of the RF amplifier 2; waveform (b) is the wave shaped signal 7, which is the output signal of the automatic gain control circuit 3; waveform (c) is the input signal to the comparator 404 in the signal presence detector 4; waveform (d) is the no-signal state gate signal 8, which is the output signal of the signal presence detector 4 and in the prior art is the gain hold signal which controls the automatic gain control circuit 3; and waveform (e) represents the gain of the automatic gain control circuit 3. Because the laser is output continuously with a high output level in the ALPC period, the waveform (a) of the amplified signal 6 obtains an extremely high read signal compared with that of the address period and data period, resulting in deterioration of the media and the frequency characteristics of the read system.
The operation in the no-signal period at the beginning of the ALPC period adjacent to the address period, and the operation in the no-signal period at the end of the ALPC period adjacent to the data period are described first.
The automatic gain control circuit 3 attempts to reduce the signal amplitude change of the amplified signal 6, but the response rate of the automatic gain control circuit 3 is unable to follow the rapid change of the no-signal period. Therefore, the wave shaped signal 7 output from the automatic gain control circuit 3 becomes a zero amplitude signal at the beginning of the no-signal period adjacent the data period. The signal presence detector 4 generates an envelope detected signal of the wave shaped signal 7. The envelope detected signal (solid line in (c)) is compared in comparator 404 with the threshold value (dot-dash line in (c)), and the no-signal state gate signal 8 (d) is set to TRUE when envelope detected signal becomes below the threshold value. During the TRUE period, the gain of automatic gain control circuit 3 is held to the predetermined value. Thus, during the no-signal periods at the beginning and ending of the ALPC period, the change of the gain level is suspended.
Operation with a signal in the period to which the laser is emitted continuously to control the laser power in the ALPC period is described next. As described above, the amplitude of the waveform (a) of the amplified signal 6 in the ALPC period is greater than that in the address period and the data period for the reasons described above. The automatic gain control circuit 3 therefore adjusts the gain level to a relatively constant level, as show by the wave shaped signal 7.
Finally, operation with a signal at the beginning of the data period is described. In the no-signal period at the end of the ALPC period adjacent to the data period, the automatic gain control circuit 3 remains set with a low gain level to hold the gain level constant by the operation described above. Therefore, at the beginning of the data period, the waveform (b) of wave shaped signal 7, which is the output of the automatic gain control circuit 3, becomes smaller. The envelope detected signal of the wave shaped signal 7 and a threshold value are compared by the comparator 404 in the signal presence detector 4, and when the input signal (solid line in (c)) of the comparator 404 is greater than the threshold value (dot-dash line in (c)), the signal presence detector 4 sets to FALSE and outputs the no-signal state gate signal 8 (d). Because the no-signal state gate signal 8, which is the gain hold control signal, is FALSE, the automatic gain control circuit 3 gradually increases the gain level to amplify the waveform (b) of the amplified signal 6 a constant amount. Conversely, when the input signal (solid line in (c)) is lower than the threshold value, the no-signal state gate signal 8 (d) remains TRUE, and the automatic gain control circuit 3 continues to hold a constant gain level. As a result, the amplitude of the amplified signal 6 waveform (b) remains low, and a stable read signal cannot, therefore, be obtained.